1. Field of the Invention
The present invention relates to a power transmitting system in a four-wheel drive vehicle including a hydraulic coupling device comprising a pair of vane pumps.
2. Description of the Prior Art
A power transmitting system in a four-wheel drive vehicle is already known from Japanese Patent Application Laid-open No. 3-104736. In this power transmitting system, an orifice which permits the communication between an intake port and a discharge port in each of the vane pumps, is formed by a small bore provided in a vane. Orifices which permit the communication between the intake ports and the discharge ports in the left and right vane pumps, are formed by clearances between a side plate partitioning rotors of the vane pumps and a cam ring.
In the above known system, since an orifice for communication between the intake port and the discharge port in each of the vane pumps, is formed by the small bore provided in the vane, the manufacturing steps required for providing the orifices in a large number of the vanes are troublesome, and further, it cannot be avoided that variations in size and shape of the orifices in the vanes are generated. For this reason, there is a possibility that the operating characteristics of the hydraulic coupling device are not stable. In addition, since the orifices for communication between the intake ports and the discharge ports in the left and right vane pumps are formed by clearances between the side plate and the cam ring, it is difficult to ensure the dimensional accuracy of the orifices, and there is a possibility that the operating characteristics of the hydraulic coupling device are not stable.